Don't you put milk in/with your tea? Do Italians really say it this way around or is this a mistake?
I think that the phrase is correct, but the translation is wrong. A correct translation would be: "My older sister often puts tea in milk".
This carries us to the eternal question: What to put first in the cup: the milk or the tea?
I agree, Mia sorella piu' grande mette latte nel tè. And remember, this sister is not bigger but older.